There are various established methods for reduction of pollutants, especially for reduction of nitric oxides, in which reducing fluids (gases or liquids) are introduced into the exhaust gas system of an internal combustion engine.
SCR technology in particular has established itself as a method of reducing nitric oxide, in which nitric oxide (NOx) contained in the nitrogen-rich exhaust is selectively reduced to nitrogen and water with the aid of ammonia or a corresponding precursor substance able to be converted into ammonia. Preferably this method is based on aqueous urea solutions. The urea solution is hydrolyzed by means of hydrolysis catalytic converters or directly at the SCR converter into ammonia and carbon dioxide. For this purpose the urea solution is injected by means of specific dosing systems upstream from the hydrolytic catalytic converter or the SCR catalytic converter into the exhaust gas stream. The problem here is to guarantee the secure supply and exact dosing of the reducing agent.
With an underdosing of the reducing agent an efficient removal of nitric oxide (NOx) from the exhaust gas cannot be achieved. An overdosing of reducing agent on the other hand can result in an undesired emission, of ammonia for example, a so-called reducing agent breakthrough.
In order to achieve a maximum conversion rate of the nitric oxide (NOx) an exact and demand-related dosing of the reducing agent is necessary, with a reducing agent breakthrough being avoided if at all possible.
With fluid reducing agents such as the widely-used urea solutions, the dosing can be undertaken using an injector. The activation time and thereby the opening time of the injector in this case are decisive for the quantity of reducing agent supplied to the exhaust gas aftertreatment system.
Ever greater injection pressures are used with modern airless SCR systems. To create the appropriate pressure the injector is connected via a line to a reducing agent pump. A high injection pressure has the advantage that the vaporization is finer and thus the ammonia can be more easily released from the mostly aqueous reducing agent solution. The ever higher pressure on the other hand means that there is an increasing danger of a leak arising at the injector and leading to an undesirably high dosing of reducing agent.
In addition, the function of the reducing agent injector means that it opens out into the exhaust and is thus subjected to the unfavorable operating conditions, such as for example the high temperatures, of the exhaust gas system. This can likewise lead to sealing problems with the injector through ageing during the service life of the dosing system.
DE 101 00 420 A1 describes a method for controlling an exhaust gas aftertreatment system for an internal combustion engine, in which a predeterminable quantity of reducing agent is supplied depending on the state of the internal combustion engine or of the exhaust gas aftertreatment system. The quantity of supplied reducing agent in this case is varied depending on the detection of an overdosing or underdosing of the reducing agent in relation to the nitric oxide throughput of the SCR catalytic converter. The method is expensive and dependent on a large number of parameters both of the internal combustion engine and also of the overall exhaust gas aftertreatment system. Manufacturing tolerances, functional changes, malfunctions or leaks in the reducing agent dosing system are not take into account here however.
The same also applies to the method for operating an internal combustion engine in accordance with DE 10 2004 046 639 A1. In this method an NOx sensor arranged downstream from the catalytic converter, which has a cross sensitivity in relation to the reagent material, interrogates a signal for predetermined operating states of the internal combustion engine which is evaluated as a measure of the reagent material slippage. However this only detects a state in which a slippage of the reagent behind the catalytic converter has already occurred. This slippage is however highly undesirable.
A method is known from DE 10 2004 018 221 A1 with which sealing problems in the entire high pressure section of the SCR system can be determined. However with this method it is not possible to trace the problem to a specific component, such as the reducing agent injector. It can thus only be established that the overall high-pressure SCR system has a sealing problem and must be completely replaced in order to rectify the problem. This is especially disadvantageous since a number of components make up the high-pressure SCR system which are distributed between a number of locations in the motor vehicle. Thus the reducing agent pump is mostly in the vicinity of the reducing agent tank, which for its part, because of the need to refill with reducing agent, must be able to be reached from outside the motor vehicle. The reducing agent injector is as a rule, to protect it from heat, not located in the vicinity of the engine and can even be arranged after any particle filter which might be present. In addition, for the purposes of pressure equalization, the high-pressure SCR system can have return lines from the reducing agent injector to the reducing agent tank. The complete removal and replacement of such an SCR system is extremely complicated and cost intensive.